1. Field of the Invention
The present invention generally relates to power amplifier circuits, and more particularly to a power amplifier circuit for an audio circuit whose distortion factor and slew rate can be improved by adding a simple circuit to a conventional power amplifier.
2. Prior Art
Conventionally, a power amplifier for an audio circuit requires a low distortion factor. In order to improve the distortion factor of the power amplifier, several methods are employed. According to a first method for improving the distortion factor, an open loop gain of the power amplifier is set as large as possible and the distortion factor thereof is set as small as possible. In addition, a feedback volume in a negative feedback loop (NFB) for the power amplifier is increased so that the NFB effects an improvement for the distortion factor.
According to a second method, a positive feedback loop (PFB) is provided to the power amplifier so that a value of an apparent gain of the power amplifier is set an infinite value. In addition, the NFB is provided to the power amplifier so that the NFB works as an ideal NFB, hence, the distortion factor is reduced.
In the above first method, in order to enlarge the open loop gain of the power amplifier, it is possible to employ a method in which the power amplifier is constituted by amplifiers (devices) having large amplification factor and another method in which the power amplifier is constituted by multistage amplifiers, for example. In the former method, there is a limitation in enlarging the open loop gain of the power amplifier because of a limitation in amplification characteristics of the devices. In the later method, as the stages of the amplifiers are increased, phase characteristics in a high frequency band of the power amplifier are deteriorated, whereby the power amplifier suffers a problem in that a transient intermodulation (TIM) distortion due to an unstable NFB and an excessive phase compensation is increased. Hence, in order to decrease the TIM distortion, there exists a limitation in enlarging the open loop gain of the power amplifier inevitably.
In the second method described before, the PFB is provided so that the distortion of the power amplifier is once increased, and the NFB reduces the increased distortion. Hence, there exists a limitation in an actual improvement effect. In addition, the second method is applied only to a primary stage portion within the power amplifier which amplifies a minute input voltage. And, the second method is used originally to reduce the increased distortion due to the PFB as small as possible and to maintain an amplification linearity of an amplifier element within the power amplifier as much as possible. Therefore, it is hard to expect that the slew rate of the power amplifier employing the second method can be improved because the PFB and NFB are provided only to the primary stage portion, and furthermore, the constitution of the power amplifier becomes complicated inevitably.